The invention relates generally to heating a window and more particularly to providing localized window heating that is sufficient for selective de-icing and defogging.
Single layer or multilayer coatings are often used to achieve desirable optical characteristics for windows used in vehicles, homes and buildings. For example, Southwall Technologies, Inc. sells a film under the federally registered trademark XIR, which is incorporated into a glass lamination to significantly reduce solar heat gain through the lamination when it is used as a window, such as an automobile windshield.
U.S. Pat. No. 6,204,480 to Woodard et al., which is assigned to the assignee of the present invention, describes the use of an optical coating on a vehicle window to heat the window for purposes of providing de-icing or defogging. The coating is a thin film stack that is electrically conductive, but is sufficiently thin to be substantially transparent. The term xe2x80x9ctransparentxe2x80x9d is defined herein as the ability to transmit at least 30 percent of radiation within the visible range of the light spectrum. Electrical connections to the thin film conductive coating are provided by bus bars. The bus bars may be patterned to achieve desired current distribution or to focus heating into certain regions of the window. The patent identifies a variety of prior art techniques for attaching bus bars to conductive glass coatings. The known techniques include using conductive enamel frits or using a border of opaque ceramic material that is bonded to an interior surface of the window along its periphery. In comparison to the prior art techniques, Woodard et al. describes using vacuum deposition approaches to forming the bus connections. A multilayer conductive optical coating is sputter deposited onto automotive windows, such as windshields. Then, bus bars are sputtered onto opposing edges of the conductive coating. As an alternative, the sputter deposition occurs on a flexible plastic substrate, such as a thin polyethylenethereph-thalate (PET) substrate, with the coated substrate then being cut to size and applied to the windshield. The patent states that it is possible to vary the thickness of the bus bars, so that the conductivity of the connection of the coating is also varied. In this manner, the heat may be focused onto certain areas of the windshield.
A concern in designing electrical systems for automobiles, such as de-icing and defogging systems, is that the systems are practically restricted to the battery power capacity of the automobile, although higher voltages may be achieved at some expense. Conventional automobile batteries provide a maximum voltage of 14 volts DC. At this voltage level, most coatings for automobile and building windows have a sheet resistance that is too high to achieve useful heating with bus-to-bus dimensions that are typical of automobile windshields and sidelights, as well as typical home and building windows. It is for this reason that bus bars are sometimes shaped to focus the heating power. U.S. Pat. No. 5,466,911 to Spagnoli et al. describes a vehicle sidelight that takes advantage of the shape of the sidelight to concentrate the heating of the portion of the window through which an operator views an exterior rearview mirror. Since the sidelight has a decreasing top-to-bottom dimension as the sidelight approaches this portion, the shape of the window inherently concentrates the heating along this portion when the bus bars are properly positioned. In order to reduce undesirable heating of the window, a non-conductive break is formed within the conductive coating below the viewable portion of the sidelight. That is, while the entirety of the exposed portion of the sidelight is heated, the heating will fluctuate on the basis of the dimensions of the sidelight and the positions of the bus bars, while the unexposed portion of the sidelight is left unheated.
Rather than heating an entire window, a de-icing or defogging system may be limited to a particular area, so that a vehicle battery voltage will result in sufficient power. U.S. Pat. No. 6,037,573 to Arsenault et al. describes forming a heating layer only within the xe2x80x9cparkxe2x80x9d region for windshield wipers. Thus, the system is able to free a wiper blade from ice. A similar system is described in U.S. Pat. No. 6,163,013 to King et al., but an electric heating grid is formed at the xe2x80x9cparkxe2x80x9d region for the windshield wipers, rather than a solid heating layer.
While the known systems operate well for their intended purposes, what is needed is a method and system for directing heating to desired locations, while controlling the localization such that excessive current does not cause damage to the components of the system.
Heating of one or more regions of a window is provided by dividing an electrically conductive and substantially transparent coating into high and low heating zones. The coating, which covers substantially the entirety of the window portion that is exposed to a viewer, is patterned to establish a preselected heating power density pattern. Power connections are then coupled to the coating such that electrical current is induced to flow through each high heating zone. In the preferred embodiment, the window is a motor vehicle window.
The heating power density pattern may be established by forming isolation traces within the transparent conductive coating. The isolation traces may merely be scribe lines from which coating material has been scratched, but more sophisticated techniques are preferred, such as the use of laser ablation or the use of localized heating/electric discharge. Since the coating is transparent, the isolation traces are largely invisible, typically only being seen in reflection.
In one application, the isolation traces are formed along a bottom of a windshield, so that the area or areas in which the windshield wipers reside is heatable. The isolation traces define a series of high-current regions along the bottom of the windshield, while the remainder of the windshield is at zero or near zero voltage. The isolation traces may be upwardly extending non-conductive lines that separate negative voltage regions from positive voltage regions. An alternating pattern of negative and positive voltage regions is preferred, but other patterns may be formed. A power source, such as a vehicle battery, is appropriately coupled to the negative voltage and positive voltage regions. For example, bus bars may be formed below the regions, effectively screening the bus bars from the view of an occupant of the vehicle.
In another application, the current flow within the heating power density pattern is not restricted to a small portion of the window. For example, a high heating zone may be substantially surrounded by low heating zones of the window, such as would be beneficial in regionally de-icing or defogging a vehicle sidelight (i.e., side window) to enable viewing of a side-mounted rearview mirror. The isolation traces within the transparent conductive coating may be located to focus current flow through a high heating zone that is aligned with the rearview mirror. A bus bar having a positive polarity is connected to one low heating zone in a position that is preferably below the window sightline for an occupant of the vehicle. Similarly, a negative polarity bus bar is connected to a second low heating zone that is preferably located below the window sightline. In order for current to flow between the two bus bars, the current must pass through the high heating zone. The coatings are identical for the low and high heating zones, but the dimensions of the high heating zone establish a high power density through the zone.
A concern is that simple terminations of the isolation traces will result in excessive current at the trace ends. The excessive current may cause damage to the coating, which is intended to provide optical benefits such as solar screening. One approach to reducing the undesired power concentration at the trace ends is to place an additional conductive element at or near the trace ends. For example, an added wire may be formed along the coating perpendicular to the ends of the isolation traces. In the application in which windshield wiper de-icing is provided, the wire may be formed horizontally along the tops of the isolation traces that separate the positive voltage regions from the negative voltage regions. Another approach to reducing the likelihood that current will cause coating and/or substrate damage is to increase the average electrical resistance at the ends of the isolation traces. For example, the coating may be patterned to include areas that require multiple current paths around the ends of the isolation traces. This may be accomplished by spot removals of coating material at the trace ends. Moreover, the trace ends may be radiused.
An advantage of the invention is that it utilizes pre-existing coatings to enable selective zone heating of a window. Zones close to a laminated window edge may be electrically heated using conductive transparent thin films with minimal heating impact in other areas of the laminate. Opposite polarity bus connections may be formed adjacent to each other, and may even be attached on the same edge of the laminate.